Dark current and defective pixel correction apparatus

ABSTRACT

A dark current correction apparatus eliminates a dark current signal component or components from an output signal of an imaging device having an array of pixels. The apparatus includes an A/D converter for digitizing the output signal, and a memory for storing level data of the digitized signal while exposure of the imaging device is controlled to be at a zero level or a uniformly illuminated level, according to whether dark current correction or white current correction, respectively, is to be performed. A dark current correction signal former generates a dark current correction signal or signals from data read out from the memory. The dark current correction signal or signals are applied to a correction circuit which processes an output signal produced by the imaging device during normal imaging operation in accordance with the correction signal or signals in order to eliminate the dark current signal component or components from that output signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to dark current correction apparatus foreliminating a dark current signal component from an output signal of animage pick-up device having a plurality of pixels, which may for examplebe arranged in an array having, for instance, a matrix configuration.

2. Description of the Prior Art

Output signals (also referred to hereinafter as "imaging outputsignals") obtained from image pick-up devices (also referred tohereinafter as "imaging devices") such as video cameras are known tosuffer from dark current arising from a number of causes, such asfluctuations in sensitivity of the imaging device or dark currenteffects, that is distortions in brightness over a wide extent of aviewing screen. For example, in a solid state imaging device, such as acharge coupled imaging device (CCD imaging device), it has been proposedto use a variety of image sensors, such as frame charge transfer,interline charge transfer or frame interline charge transfer type imagesensors, according to which signal charges from pixels of the sensor,which are arranged in a matrix configuration, are transferred in thevertical direction and are sequentially read out by means of ahorizontal transfer register, so that one horizontal line is read outduring one horizontal scanning period, and the signal charges for thetotality of the pixels of the device (and therefore for the totality ofthe pixels of a field or frame) are read out during one verticalscanning period, so as to produce an imaging output signal. Thus, a darkcurrent proportional to the time during which the signal charges aretransferred to the horizontal transfer register is added to the signalcharges to give rise to sawtooth (serrated) changes in brightness duringeach vertical scanning period, that is dark current in the verticaldirection. On the other hand, the dark current in the horizontaltransfer register gives rise to sawtooth (serrated) changes inbrightness during each horizontal scanning period, that is dark currentin the horizontal direction.

In general, the above-mentioned dark current may be classified intowhite (modulation) according to which the output is lowered inperipheral regions of the viewing screen, and black (superposition)according to which the black level is not uniform throughout the viewingscreen. Dark current correction processing is conventionally performedby mixing dark current correction signals into the imaging outputsignals in an analog fashion, using a multiplier for white and an adderfor black. The dark current correction signals may be formed bygenerating sawtooth and parabolic signals for the horizontal andvertical directions, and combining these signals.

With the conventional dark current correction circuit, output levels ofthe sawtooth signal generators and the parabolic signal generators maybe manually adjusted by means of a level control means, such as a volumeknob. Thus, the output levels of the signal generators are adjustedmanually to achieve optimum dark current correcting processing whilereference is continuously made to a waveform monitor. U.S. Pat. No.4,731,652, issued Mar. 15, 1988 to Yamanaka, discloses such anarrangement in which the levels of output signals of horizontal andvertical sawtooth and parabolic waveform generators are manuallyadjustable by means of gain control circuits comprising variableresistors.

If use is made of a three color CCD imaging system in which an objectimage is separated into color components, for example red, green andblue components, and images of the three color components are formedseparately by three respective imaging devices, dark current correctionhas to be performed for each of the imaging devices.

Since, with the conventional dark current correction circuit, darkcurrent correction is achieved by manually adjusting the output levelsof the sawtooth and parabolic signal generators while continuouslyreferring to a waveform monitor, the problem arises that atime-consuming adjustment operation by a skilled operator is needed forachieving accurate adjustment. This problem is serious even in the caseof a single output imaging device. The problem is even more serious inthe case of the three color CCD imaging system, in that the dark currentcorrection that has to be performed for each of the three imagingdevices is even more labor-intensive and time-consuming.

SUMMARY AND OBJECTS OF THE INVENTION First Form of Implementation

A first object of the invention is to provide a dark current correctionapparatus for eliminating (wholly or partially) a dark current signalcomponent from an imaging output signal of one or more imaging deviceshaving a plurality of pixels (arranged for instance in a matrixconfiguration), which enables satisfactory dark current correction to beachieved quickly and reliably.

According to one aspect of a first form of implementation thereof,directed to achieving the first object mentioned above, the inventionprovides a dark current, correction apparatus in which dark current,correction data for use in dark current correction are formed from animaging output signal obtained from an imaging device and are stored inmemory means, the dark current correction data are read out from thememory means during actual imaging, and dark current correction of theimaging output signal of the imaging device is automatically performedby correction means with the aid of a dark current correction signalformed by correction signal forming means on the basis of the storeddark current correction data.

According to another aspect of the first form of implementation thereof,the invention provides a dark current correction apparatus foreliminating (partially or wholly) dark current signal components fromimaging output signals of plural imaging devices each having an array ofa large number of pixels in a matrix configuration, wherein level dataof imaging output signal portions corresponding to a predeterminednumber of the pixels are stored in storage means as dark currentcorrection data, those imaging output signals of the imaging deviceshaving been produced under light exposure control by light exposurecontrol means and digitized by analog/digital converters, dark currentcorrection signals are formed by dark current correction signal formingmeans on the basis of the dark current correction data read out from thestorage means, and imaging output signals of the imaging devices areautomatically processed by dark current correction means during actualimaging on the basis of dark current correction signals supplied bycorrection signal forming means.

According to a further aspect of the first form of implementationthereof, the invention provides a dark current apparatus for eliminatingdark current signal components of imaging output signals of imagingdevices each having a large number of pixels arranged in a matrixconfiguration, the apparatus comprising light exposure control means forthe imaging devices, analog to digital converters for digitizing imagingoutput signals of the imaging devices, storage means for storing leveldata of the imaging output signals of a predetermined number of thepixels as dark current correction data, those imaging output signals ofthe imaging devices having been produced under light exposure control bythe light exposure control means and digitized by the analog to digitalconverters, dark current correction signal forming (generating) meansfor forming dark current correction signals on the basis of the darkcurrent correction data read out from the storage means, and correctionmeans for subjecting imaging output signals of the imaging devices todark current correction processing on the basis of the dark currentcorrection signals formed by the correction signal forming means duringimaging. The output signals of the analog to digital converters are thusavailable to downstream signal processing circuits as dark currentcorrected imaging output signals.

The stored dark current correction data may be derived from imagingoutput signals produced by the imaging devices under a condition inwhich no light is incident on imaging surfaces of the imaging devices,whereby dark current correction signals are formed by the correctionsignal forming means on the basis of the dark current correction dataread out from the storage means. The correction means may then subjectthe imaging output signals to dark current correction by subtracting thedark current correction signals from imaging output signals of theimaging devices.

Additionally or alternatively, the stored dark current correction datamay be derived from imaging output signals produced by the imagingdevices with light of uniform light intensity or volume incident on theentirety of the imaging surfaces of the imaging devices, whereby darkcurrent correction signals are formed by the correction signal formingmeans on the basis of the dark current correction data read out from thestorage means. The correction means may then subject the imaging outputsignals to white shading correction by dividing the imaging outputsignals of the imaging devices by the dark current correction signals.

The storage means may comprise a random access memory and anelectrically erasable programmable read-only memory, and the outputsignals of the analog to digital converters may be stored in theprogrammable read-only memory at a reduced data volume so that thenumber of the output data is less at the middle than at the margin(periphery) of each of the imaging surfaces. The random access memorymay be used for forming the dark current correction data or correctingthe dark current on the basis of the dark current correction data,whereas the electrically erasable programmable read-only memory thereofmay be used for prolonged storage of the dark current correction data.

Second Form of Implementation

A second object of the invention is to provide a dark current correctionapparatus for performing dark current correction of an imaging outputsignal from an imaging device by means of a sawtooth signal generated bya sawtooth signal generating means and a parabolic signal generated by aparabolic signal generating means, in which optimum dark currentcorrection processing may be realized quickly and reliably.

A third object of the invention is to provide a dark current correctionapparatus in which sawtooth and parabolic signals having signal levelsnecessary for effecting dark current correction are formed automaticallyon the basis of an imaging output signal obtained from the imagingdevice, in order that a satisfactory dark current correcting operationmay be performed automatically.

According to one aspect of a second form of implementation thereof,directed to achieving the second and third objects mentioned above, theinvention provides a dark current correction apparatus in which animaging output signal from an imaging device is subjected to darkcurrent correction processing by means of sawtooth (serrated) signalsfrom sawtooth (serrated) signal generating means and parabolic signalsfrom parabolic signal generating means, level data of an imaging outputsignal of pixels of the imaging device produced with no light incidenton the imaging device, or with light of uniform light intensity incidenton the whole of an imaging surface of the imaging device, are sampled atpredetermined intervals in the vertical and horizontal directions toproduce first and second data strings, coefficients of the sawtoothsignals and parabolic signals used for dark current correctionprocessing are calculated on the basis of the first and second datastrings and, using these coefficients, the signal levels of the sawtoothsignals and parabolic signals are controlled during imaging in orderautomatically to perform satisfactory dark current correctingprocessing.

According to another aspect of the second form of implementationthereof, the invention provides a dark current correction apparatus foreliminating (partially or wholly) a dark current signal component froman imaging output signal produced by an imaging device having an arrayof a large number of pixels arranged in a matrix configuration, theapparatus comprising an analog to digital converter for digitizing animaging output signal from the imaging device, sampling means forsampling level data of an imaging output signal for pixels of saidimaging device at predetermined intervals in the vertical and horizontaldirections in order to produce first and second data strings, thatimaging output signal having been produced and digitized by the analogto digital converter with no light incident on the imaging device, orwith light of a uniform light intensity incident on all of an imagingsurface of the imaging device, arithmetic means for finding first andsecond quadratic curves on the basis of the first and second datastrings outputted from the sampling means, sawtooth signal generatingmeans for generating a sawtooth signal, parabolic signal generatingmeans for generating a parabolic signal, a first level control circuitfor controlling the signal level of the parabolic signal on the basis ofcoefficients of the second order terms of the first and second quadraticcurves found by the arithmetic means, a second level control circuit forcontrolling the signal level of the sawtooth signal on the basis ofcoefficients of the first order terms of the first and second quadraticcurves found by the arithmetic means, and correction means forsubjecting an imaging output signal from the imaging device to darkcurrent correction processing by the parabolic and sawtooth signals ascontrolled in signal level by the first and second level controlcircuits. Output data of the analog to digital converter are thusavailable to downstream signal processing circuitry as dark currentcorrected imaging output data.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of this inventionwill be apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram showing the construction of a dark currentcorrection apparatus according to a first embodiment of the invention;

FIG. 2 is an illustrative view showing the arrangement of an array ofpixels of a solid-state imaging device supplying an imaging outputsignal to the dark current correction apparatus of FIG. 1, and typicalshading characteristics thereof in the horizontal and verticaldirections;

FIG. 3 is an illustrative view showing data strings of black darkcurrent correction data and white dark current correction data stored ina memory of the shading correction apparatus of FIG. 1;

FIG. 4 is a block diagram showing the detailed construction of a dataprocessing circuit of a dark current correction signal forming sectionof the dark current correction apparatus of FIG. 1;

FIG. 5 is a flow chart showing a control procedure carried out by asystem controller of the dark current correction apparatus of FIG. 1;

FIGS. 6a and 6b are waveform diagrams showing a dark current correctionsignal formed by the dark current correction signal forming section ofthe apparatus of FIG. 1;

FIG. 7 is a block diagram showing the construction of a dark currentcorrection apparatus according to a second embodiment of the invention;

FIG. 8 is an illustrative view showing an array of pixels of asolid-state imaging device supplying an imaging output signal to thedark current correction apparatus of FIG. 7, and exemplary dark currentcharacteristics thereof in the horizontal and vertical directions;

FIG. 9 is a block diagram showing an exemplary sawtooth signalgenerating circuit of a digital circuit configuration that can beemployed in the dark current correction apparatus of FIG. 7;

FIG. 10 is a graph showing output characteristics for use in describingthe operation of the sawtooth signal generating circuit shown in FIG. 9;

FIG. 11 is a block diagram showing an exemplary parabolic signalgenerating circuit of a digital circuit configuration that can beemployed in the dark current correction apparatus of FIG. 7; and

FIG. 12 is a graph showing output characteristics for use in describingthe operation of the parabolic signal generating circuit shown in FIG.11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A dark current correction apparatus according to a first embodiment ofthe invention will now be described in detail with reference to FIGS. 1to 6 of the drawings. In this specification, "dark current" and"defective pixel" corrections are used interchangeably.

In the present embodiment, the invention is applied by way of example toa three color CCD imaging device. As shown in FIG. 1, imaging outputsignals E_(R), E_(G) and E_(B) produced by first, second and thirdimaging devices 1R, 1G and 1B for R, G and B (red, green and blue)channels, respectively, are supplied to a correction circuit 3 by meansof preamplifiers 2R, 2G and 2B, respectively. The imaging devices 1R, 1Gand 1B constitute an imaging section of the above-mentioned three colorCCD imaging device, which is provided in an imaging optical system 7including an imaging lens 4, an iris unit or device 5 and a colorseparation prism 6. Each of the imaging devices 1R, 1G and 1B is a CCDimage sensor formed by a number (M×N) of pixels S₁₁ to S_(MN) (FIG. 2)arranged in a matrix array comprising a number M of pixels in thehorizontal direction and a number N of pixels in the vertical direction,as shown in FIG. 2, and is driven by a CCD driver (not shown) so thatsignal charges from the totality of the pixels S₁₁ to S_(MN) for eachfield or frame will be read out during one vertical scanning period.

The red color component of the imaging output signal E_(R) of an objectimage, which has been color-separated by the color separating prism 6,is supplied from the first imaging device 1R to the correction circuit 3by way of the preamplifier 2R as an R-channel signal. The green colorcomponent of the image output signal E_(G) of the object image, whichhas been color-separated by the color separating prism 6, is suppliedfrom the second imaging device 1G to the correction circuit 3 by way ofthe preamplifier 2G as a G-channel signal. Similarly, the blue colorcomponent of the image output signal E_(B) of the object image, whichhas been color-separated by the color separating prism 6, is suppliedfrom the third imaging device 1B to the correction circuit 3 by way ofthe preamplifier 2B as a B-channel signal.

The correction circuit 3 processes the imaging output signals E_(R),E_(G) and E_(B), produced by the imaging devices 1R, 1G and 1B for theR, G and B channels, respectively, by effecting black dark currentcorrection processing and white dark current correction processing. Thecorrection circuit 3 includes subtractors 8R, 8G and 8B for the R, G andB channels, supplied with the imaging output signals E_(R), E_(G) andE_(B), respectively, and dividers 10R, 10G and 10B for the R, G and Bchannels, supplied with subtraction output signals from the subtractors8R, 8G and 8B, by means of variable gain preamplifiers 9R, 9G and 9B,respectively.

In the correction circuit 3, the subtractors 8R, 8G and 8B process theimaging output signals E_(R), E_(G) and E_(B) of the R, G and Bchannels, respectively, by effecting black dark current correctionprocessing by subtracting from the imaging output signals E_(R), E_(G)and E_(B) black dark current correction signals B_(RSH), B_(GSH) andB_(BSH) for the R, G and B channels, respectively, that are suppliedfrom a dark current correction signal forming section 14 by which theyare generated as described hereinbelow. The variable gain amplifiers 9R,9G and 9B process the imaging output signals E_(R), E_(G) and E_(B) ofthe R, G and B channels by signal level adjustment, such as whitebalance or black balance adjustment, and are gain-controlled by controlsignals for the R, G and B channels supplied from a system controller 27described hereinbelow. The dividers 10R, 10G and 10B process the imagingoutput signals E_(R), E_(G) and E_(B) of the R, G and B channels,respectively, by effecting white dark current correction processing bydividing the imaging output signals E_(R), E_(G) and E_(B) by white darkcurrent correction signals W_(RSH), W_(GSH) and W_(BSH) for the R, G andB channels, respectively, that are supplied from the dark currentcorrection signal forming section 14.

The dividers 10R, 10G and 10B may be implemented as multipliersoperative to multiply the imaging output signals E_(R), E_(G) and E_(B)of the R, G and B channels by reciprocals of the white dark currentcorrection signals W_(RSH), W_(GSH) and W_(BSH), respectively.

The imaging output signals E_(R), E_(G) and E_(B), corrected by thecorrection circuit 3, are supplied therefrom to analog to digital (A/D)converters 12R, 12G and 12B of the R, G and B channels, respectively,via respective preknee circuits 11R, 11G and 11B.

The preknee circuits 11R, 11G and 11B effect non-linear processing ofthe imaging output signals E_(R), E_(G) and E_(B) of the R, G and Bchannels outputted by the correction circuit 3 in order to prevent theinput signal levels to the A/D converters 12R, 12G and 12B exceeding thedynamic range thereof.

The A/D converters 12R, 12G and 12B provide digital level dataindicating the signal levels of the imaging output signals E_(R), E_(G)and E_(B) as corrected by the correction circuit 3. Level dataindicating the levels of the imaging output signals E_(R), E_(G) andE_(B) of the R, G and B channels, provided by the A/D converters 12R,12G and 12B, respectively, are supplied as dark current correctedconcurrent imaging output data D_(R), D_(G) and D_(B) to the darkcurrent correction signal forming section (also referred to hereinafteras "the dark current correction signal former") 14, and to downstreamsignal processing circuits (not shown), via defect correction circuits13R, 13G and 13B, respectively.

The defect correction circuits 13R, 13G and 13B process signal chargesfrom defective pixels of the imaging devices 1R, 1G and 1B, that isparts of the imaging output signals E_(R), E_(G) and E_(B) correspondingto defective pixels of the R, G and B channels, by defect correctionprocessing that corrects their signal levels on the basis of level dataof previously detected defective pixels.

The dark current correction signal former 14 includes: low-pass filters(LPFs) 15R, 15G and 15B supplied with the concurrent imaging output dataD_(R), D_(G) and D_(B) of the channels R, G and B, respectively; a dataselector 16 supplied with the concurrent imaging output data D_(R),D_(G) and D_(B), respectively, by means of the low-pass filters 15R, 15Gand 15B; a data processing circuit 17 supplied with dot sequential dataD[R/G/B] selected by the data selector 16; a working memory 18, which isan overwritable random access memory (RAM), connected to the dataprocessing circuit 17; a backup memory 19, which is an electricallyerasable and programmable read-only memory (EEPROM), similarly connectedto the data processing circuit 17; a data selector 20 for distributingblack correction data D[B_(RSH) /B_(GSH) /B_(BSH) ], outputteddot-sequentially from the data processing circuit 17, to the R, G and Bchannels; a data selector 21 for distributing white shading correctiondata D[W_(RSH) /W_(GSH) /W_(BSH) ], outputted dot-sequentially from thedata processing circuit 17, to the R, G and B channels; digital toanalog (D/A) converters 23R, 23G and 23B for converting the black darkcurrent correction data D[B_(RSH) ], D[B_(GSH) ] and D[B_(BSH) ],distributed by the data selector 20, into corresponding analog signals;D/A converters 24R, 24G and 24B for converting the white dark currentcorrection data D[W_(RSH) ], D[W_(GSH) ] and D[W_(BSH) ] for the R, Gand B channels, as distributed by the data selector 21, intocorresponding analog signals; and low-pass filters 25R, 25G, 25B, 26R,26G and 26B provided on the output sides of the D/A converters 23R, 23G,23B, 24R, 24G and 24B, respectively.

The low-pass filters 15R, 15G and 15B of the dark current correctionsignal former 14 are digital filters having a cut-off frequency equal toone-eighth of a clock frequency of the A/D converters 12R, 12G and 12B,and limit the bandwidths of the concurrent imaging output data D_(R),D_(G) and a D_(B) to one-eighth by performing bandwidth limitation.

The data selector 16 selects the concurrent imaging output data D_(R),D_(G) and D_(B) of the channels R, G and B, as thus bandwidth-limited bythe low-pass filters 15R, 15G and 15B, dot-sequentially on achannel-by-channel basis, thereby forming the dot-sequential dataD[R/G/B] with the number of data reduced to one-eighth of the originalnumber. The dot-sequential data D[R/G/B], thus formed by data selector16, indicate dot-sequentially the signal levels of the imaging outputsignals from signal charges of every eighth pixel of the pixels S₁₁ toS_(MN) of the imaging devices 1R, 1G and 1B, namely those shown hatchedin FIG. 2.

It is to be noted that a driver or control circuit 28 of the iris unit 5of the imaging optical system 7 is so controlled by the systemcontroller 27 that: for detecting the black dark currentcharacteristics, the iris unit 5 is closed and the imaging devices 1R,1G and 1B perform an imaging operation without any light falling ontheir respective imaging surfaces; and, for detecting the white darkcurrent characteristics, the iris unit 5 is opened and the imagingdevices 1R, 1G and 1B perform an imaging operation with light of auniform light intensity corresponding to 100 percent brightness fallingon the entirety of the imaging surfaces.

The data processing circuit 17 finds black dark current correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ] and white dark current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ], consistent with the black and white darkcurrent characteristics of the imaging devices 1R, 1G and 1B, on thebasis of the dot-sequential data D[R/G/B] supplied from the dataselector 16, and stores the data dot-sequentially in the working memory18 as shown in FIG. 3. During actual imaging, the data processingcircuit 17 reads out the black dark current correction data D[B_(RSH)/B_(GSH) /B_(BSH) ] and the white dark current correction data D[W_(RSH)/W_(GSH) /W_(BSH) ] from the working memory 18 dot-sequentially so thatthe thus read-out data can be outputted by means of the data selectors20 and 21.

In the present embodiment, the data processing circuit 17 processes thedot-sequential data D[R/G/B], indicating dot-sequentially the signallevels of the imaging output signals from signal charges at every eighthpixel of the pixels S₁₁ to S_(MN) of the imaging devices 1R, 1G and 1B,by: integrating the level data indicative of the imaging output levelsof the pixels lying at the same horizontal positions P_(h1) to P_(hm) asshown in FIG. 2 to produce a data strong D[1_(h1) to 1_(hm) ] indicatingthe horizontal dark current characteristics with the use of level datahaving a thereby improved signal to noise (S/N) ratio to producehorizontal dark current correction data dot-sequentially from the datastring D[1_(h1) to 1_(hm) ]; and by integrating the level dataindicative of the imaging output levels of the pixels lying at the samevertical positions P_(v1) to P_(vm) to produce a data string D[1_(v1) to1_(vm) ] indicating the vertical dark current characteristics with theuse of level data having a thereby improved S/N ratio to producevertical dark current correction data dot-sequentially from the datastring D[1_(v1) to 1_(vm) ].

The data processing circuit 17 may for example be constructed as shownin FIG. 4. Thus, the dot-sequential data D[R/G/B] from the data selector16 are supplied to a clipping circuit 31. The clipping circuit 31processes the dot-sequential data D[R/G/B] by dot-sequential subtractionof the mean values of the respective frames of the imaging devices 1R,1G and 1B and clipping to the lower n bits. The clipped dot-sequentialdata D[R/G/B] are supplied to a down-sampling circuit 32 which processesthe clipped dot-sequential data D[R/G/B] by limiting the bandwidth ofthe dot-sequential data D[R/G/B] indicating the dark currentcharacteristics of the imaging devices 1R, 1G and 1B to one sixteenth byusing, for example, a digital filter having a transfer function H(z)given by:

    H(z)=1/4Z.sup.-4 +1/2Z.sup.0 +1/4Z.sup.4.

The dot-sequential data D[R/G/B], thus down-sampled by the down-samplingcircuit 32, are supplied to an accumulator or register 33. Theaccumulator 33 processes the dot-sequential data D[R/G/B] byconcurrently adding and integrating the level data indicating theimaging output levels of the pixels at the same horizontal positionsP_(h1) to P_(hn), using the working memory 18, as shown in FIG. 2,thereby producing black dark current correction data D[B_(RSH) /B_(GSH)/B_(BSH) ]_(H) and white dark current correction data D[W_(RSH) /W_(GSH)/W_(BSH) ]_(H) dot-sequentially as dark current correction dataconsistent with the horizontal component for each of the imaging devices1R, 1G and 1B. The accumulator 33 also processes the dot-sequential dataD[R/G/B] by integrating the level data indicating the imaging outputlevels of the pixels at the same horizontal positions P_(v1) to P_(vm),using the working memory 18, thereby producing black correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) and white dark current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ]_(V) dot-sequentially as dark currentcorrection data consistent with the vertical shading component for eachof the imaging devices 1R, 1G and 1B. It will be noted that the blackdark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and thewhite dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(H) inthe horizontal direction are formed by concurrent addition in theaccumulator 33, while the black dark current correction data D[B_(RSH)/B_(GSH) /B_(BSH) ]_(V) and the white dark current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ]_(V) in the vertical direction are formedby concurrent addition in memory, using the working memory 18.

In this manner, the black dark current correction data D[B_(RSH)/B_(GSH) /B_(BSH) ]_(H) and the white dark current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ]_(H) in the horizontal direction, as wellas the black dark current correction data D[B_(RSH) /B_(GSH) /B_(BSH)]_(V) and the white dark current correction data D[W_(RSH) /W_(GSH)/W_(BSH) ]_(V) in the vertical direction, for the imaging devices 1R, 1Gand 1B, formed from the dot-sequential data D[R/G/B], are written andstored dot-sequentially in the working memory 18.

The black dark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H)and the white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH)]_(H) in the horizontal direction, as well as the black dark currentcorrection data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) and the white darkcurrent correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(V) in thevertical direction, thus stored in the working memory 18, are read outdot-sequentially from the working memory 18 and supplied via a buffer 34to a horizontal/vertical (H/V) data separator 35 when performing darkcurrent correction of the imaging output signals of the R, G and Bchannels.

The data separator 35 separates the black dark current correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and the white correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ]_(H) for the horizontal direction and theblack dark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) andthe white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(V)for the vertical direction, and transmits the black dark currentcorrection data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and the white darkcurrent correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(H) for thehorizontal direction to an interpolation circuit 36, while transmittingthe black current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) andthe white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(V)to adders 37 and 38, respectively.

The interpolation circuit 36 processes the black dark current correctiondata D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and the white dark currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(H), supplied thereto fromthe data separator 35 dot-sequentially at a data rate equal to oneeighth of the clock frequency, by mean value interpolation, whileseparating the black dark current correction data D[B_(RSH) /B_(GSH)/B_(BSH) ]_(H) and the white shading correction data D[W_(RSH) /W_(GSH)/W_(BSH) ]_(H) from each other and outputting these separated data at adata rate of 1/4.

The horizontal black dark current correction data D[B_(RSH) /B_(GSH)/B_(BSH) ]_(H), obtained at the data rate of 1/4 from the interpolationcircuit 35, are supplied to the adder 37. The adder 37 adds thehorizontal black dark current correction data D[B_(RSH) /B_(GSH)/B_(BSH) ]_(H) to the vertical black dark current correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) to form horizontal and vertical blackdark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ] which arethen outputted by means of a clipping circuit 39.

The horizontal white dark current correction data D[W_(RSH) /W_(GSH)/W_(BSH) ]_(H), outputted at the data rate of 1/4 by the interpolatingcircuit 36, are supplied to the adder 38. The adder 38 adds thehorizontal white dark current correction data D[W_(RSH) /W_(GSH)/W_(BSH) ]_(H) and the vertical white shading correction data D[W_(RSH)/W_(GSH) /W_(BSH) ]_(V) to each other to produce horizontal and verticalwhite dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ], whichare then outputted by means of the clipping circuit 39.

Referring back to FIG. 1, the data selector 20, supplied with the blackdark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]dot-sequentially outputted from the data processing circuit 17, isoperative to distribute the black dark current correction data D[B_(RSH)/B_(GSH) /B_(BSH) ] to the D/A converters 23R, 23G and 23B of the R, Gand B channels, and is constituted by, for example, a latch circuit. Theblack dark current correction data D[B_(RSH) ], D[B_(GSH) ] andD[B_(BSH) ] supplied from the selector 20 are converted into analogsignals by the D/A converters 23R, 23G and 23B, respectively.

The output signal from the D/A converter 23R for converting the blackdark current correction data D[B_(RSH) ] into an analog signal aresupplied via the low-pass filter 25R to the R-channel subtractor 8R ofthe correction circuit 3 as the black dark current correction signalB_(RSH). The output signal from the D/A converter 23G for converting theblack dark current correction data D[B_(GSH) ] into an analog signal aresupplied via the low-pass filter 25G to the G-channel subtractor 8G ofthe correction circuit 3 as the black dark current correction signalB_(GSH). Similarly, the output signal from the D/A converter 23B forconverting the black dark current correction data D[B_(BSH) ] into ananalog signal are supplied via the low-pass filter 26B to the B-channelsubtractor 8B of the correction circuit 3 as the black dark currentcorrection signal B_(BSH).

The data selector 21, supplied with the white dark current correctiondata D[W_(RSH) /W_(GSH) /W_(BSH) ] dot-sequentially outputted from thedata processing circuit 17, is operative to distribute the white darkcurrent correction data D[W_(RSH) /W_(GSH) /W_(BSH) ] to the D/Aconverters 24R, 24G and 24B of the R, G and B channels, and isconstituted by, for example, a latch circuit. The D/A converters 24R,24G and 24B are operative to convert the white dark current correctiondata D[B_(RSH) ], D[B_(GSH) ] and D[B_(BSH) ] from the data selector 21into analog signals.

The output signal of the D/A converter 24R for converting the white darkcurrent correction data D[W_(RSH) ] into an analog signal are suppliedvia the low-pass filter 26R to the R-channel divider 8R of thecorrection circuit 3 as the white dark current correction signalW_(RSH). The output signal of the D/A converter 24G for converting thewhite dark current correction data [W_(GSH) ] into an analog signal aresupplied via the low-pass filter 26G to the G-channel divider 8G of thecorrection circuit 3 as the white dark current correction signalW_(GSH). Similarly, the output signal of the D/A converter 24B forconverting the white dark current correction data D[W_(BSH) ] into ananalog signal are supplied via the low-pass filter 26B to the B-channeldivider 8B of the correction circuit 3 as the white dark currentcorrection signal W_(BSH).

The shading correction apparatus of the present embodiment is controlledby the system controller 27 in a manner which will now be described withreference to a flow chart shown in FIG. 5.

Thus, when a dark current detection mode is entered, the operation ofdetecting the black dark current characteristics is initiated. At afirst step S₁, the iris unit 6 is closed and the imaging devices 1R, 1Gand 1B thus perform an imaging operation without any light falling ontheir imaging surfaces.

At the next (second) step S₂, the black dark current correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ] in the working memory 18 are all set to 0,while the white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH)] are all set to 1.

At the next (third) step S₃, the data processing circuit 17 processesthe imaging output signals E_(R), E_(G) and E_(B), produced by theimaging devices 1R, 1G and 1B without any light incident on theirimaging surfaces, for forming black dark current correction dataD[B_(RSH) /B_(GSH) /B_(BSH) ] on the basis of the dot-sequential dataD[R/G/B] and dot-sequentially storing the data in the working memory 18.

At the next (fourth) step S₄, the data processing circuit 17 reads outthe black dark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ] andthe white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]dot-sequentially from the working memory 18 and processes the imagingoutput signals E_(R), E_(G) and E_(B) from the imaging devices 1R, 1Gand 1B by effecting dark current correction processing by means of thecorrection circuit 3. The data processing circuit 17 then detects blackshading correction errors of the dark current corrected imaging outputsignals E_(R), E_(G) and E_(B) by, for example, a least squares method.

At the next (fifth) step S₅, it is determined whether the black darkcurrent correction errors of the dark current corrected imaging outputsignals E_(R), E_(G) and E_(B) detected at the fourth step S₄ are notlarger than a predetermined value (threshold). If the result of thedecision made at the fifth step S₅ is NO, that is if the dark currentcorrection errors are larger than the predetermined value, the systemcontroller 27 proceeds to a step S₆ to control the gains of the variablegain amplifiers 9R, 9G and 9B of the correction circuit 3 in a directionto reduce the dark current correction errors. The system controller thenreverts to the step S₂ to repeat the operations from the step S₂ to thestep S₆. If the result of the decision made at the fifth step S₅ is YES,that is if the dark current correction errors are not larger than thepredetermined value, the operation of detecting the black dark currentcharacteristics is terminated when the dark current corrected imagingoutput signals E.sub. R, E_(G) and E_(B) fall to below the predeterminedvalue. The system controller 27 then proceeds to a seventh step S₇.

At the seventh step S₇, the system controller 27 decides whetherdetection of the white dark current characteristics is to be performed.If the result of the decision is NO, that is if detection of the whitedark current characteristics is not to be performed, the controloperation of the detection mode of detecting the white dark currentcharacteristics is terminated. If the result of the decision at theseventh step S₇ is YES, that is if detection of the white dark currentcharacteristics is to be performed, the system controller 27 proceeds tothe next (eighth) step S₈.

At the eighth step S₈, the iris unit 6 is opened and the imaging devices1R, 1G and 1B perform an imaging operation with light of uniform lightintensity corresponding to 100 percent brightness falling on the wholeof the imaging surfaces with the use of a white color pattern, such as aPorta pattern.

At the next (ninth) step S₉, the data processing circuit 17 processesthe imaging output signals E_(R), E_(G) and E_(B) produced by theimaging devices 1R, 1G and 1B with light of uniform light intensitycorresponding to 100 percent brightness falling on the whole of theimaging surfaces, for forming white dark current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ] on the basis of the dot-sequential dataD[R/G/B] and dot-sequentially storing the data in the working memory 18.

At the next (tenth) step S₁₀, the data processing circuit 17 reads outthe black dark current correction data D[B_(RSH) /B_(GSH) /B_(BSH) ] andthe white dark current correction data D[W_(RSH) /W_(GSH) /W_(BSH) ]from the working memory 18 dot-sequentially and processes the imagingoutput signals E_(R), E_(G) and E_(B) from the imaging devices 1R, 1Gand 1B by dark current correction processing effected by the correctioncircuit 3, while detecting white dark current correction errors of theshading corrected imaging output signals E_(R), E_(G) and E_(B) by, forexample, a least squares method.

At the next step S₁₁, it is determined whether the white dark currentcorrection errors of the dark current corrected imaging output signalsE_(R), E_(G) and E_(B) detected at the tenth step S₁₀ are not largerthan a predetermined value (threshold). If the result of the decisionmade at the step S₁₁ is NO, that is if the white dark current correctionerrors are larger than the predetermined value, the system controllerproceeds to a step S₁₂ to control the gains of the variable gainamplifiers 9R, 9G and 9B of the correction circuit 3 in a direction todecrease the white dark current correction errors. Then, at the nextstep S₁₃, the system controller 27 sets all the white shading correctiondata D[W_(RSH) /W_(GSH) /W_(BSH) ] in the working memory 18 to 1, afterwhich the system controller reverts to the step S₉ to repeat theoperations from the step S₉ to the step S₁₃. If the result of thedecision made at the step S₁₁ is YES, that is if the white dark currentcorrection errors of the shading corrected imaging output signals E_(R),E_(G) and E_(B) are not larger than the predetermined value, theoperation of detecting the white dark current characteristics isterminated. The system controller 27 proceeds to a steps S₁₄ to performwhite balance adjustment to terminate the control operation of thedetection mode for detecting the dark current characteristics.

At the step S₁₄, with the imaging output signals E_(R), E_(G) and E_(B)of the R, G and B channels, produced by the imaging devices 1R, 1G and1B and white dark current corrected on the basis of the black shadingcorrection data D[B_(RSH) /B_(GSH) /B_(BSH) ] and the white dark currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ] fetched dot-sequentiallyinto the working memory 13, as described previously, the systemcontroller 27 sets the gains of the variable gain amplifiers 9R, 9G and9B of the correction circuit 3 so that the imaging output concurrentdata D_(R), D_(G) and D_(B) of the respective channels will exhibit anequal signal level, thereby performing white balance adjustment.

The operation of detecting the dark current characteristics without anylight being incident on the imaging surfaces of the imaging devices 1R,1G and 1B, that is the black dark current characteristics, may beperformed at any desired occasion of closing the iris unit 5. However,the operation of detecting the white dark current characteristics cannotbe performed so frequently, because the imaging operation has to beperformed with light of uniform light intensity with 100 percentbrightness falling on the imaging surfaces of the imaging devices 1R, 1Gand 1B with the use of a white color pattern, such as a Porta pattern.Therefore, the latest white dark current correction data as found by thedetecting operation of the white dark current characteristics arepreferably stored in the backup memory (EEPROM) 19.

In general, the dark current characteristics of an imaging device aresubject to larger changes in the marginal (peripheral) region of theimaging surface than at the middle of the imaging surface. For thisreason, data may be eliminated periodically, so that the data volume islesser at the middle of the imaging surface, for conserving the storagecapacity of the backup memory 19. For example, the horizontal white darkcurrent correction data D[W_(RSH) /W_(GSH) W_(BSH) ]_(H) may besubjected to down-sampling in such a manner that the number of data isreduced by factors of 1/8 and 1/128 at the margin and at the middle ofthe imaging surface, respectively, while the vertical white dark currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(V) may be subjected todown-sampling in such a manner that the number of data is reduced byfactors of 1/4 and 1/32 at the margin and at the middle of the imagingsurface, respectively.

In the present embodiment, the data processing circuit 17 is providedwith a processing circuit 40 (FIG. 4) whereby the latest white darkcurrent correction data D[W_(RSH) /W_(GSH) /W_(BSH) ] as found by theabove-described detection operation for detecting the white dark currentcharacteristics are read out from the working memory 18 and subjected todown-sampling to reduce the number of data stored in the backup memory19 in that the white dark current correction data with the lesser datavolume read out from the buffer memory 19 are interpolated and writtenin the working memory 18 via a buffer circuit 41 as the white darkcurrent correction data.

The data reduction and interpolation effected in the processing circuit40 may be implemented by a digital filter having a transfer functionH(z) given by:

    H(z)=1/4Z.sup.-4 +1/2Z.sup.0 +1/4Z.sup.4.

for sequentially providing the data rates through the filter equal to1/2 or 2 times.

It should be noted that, in the above-described dark current correctionsignal former 13, optimum correction occasionally cannot be achieved bythe black dark current correction signals B_(RSH), B_(GSH) and B_(BSH)of the R, G and B channels, supplied from the D/A converters 23R, 23Gand 23B to the correction circuit 3 by way of the low-pass filters 25R,25G and 25B, or by the white dark current correction signals W_(RSH),W_(GSH) and W_(BSH) supplied from the D/A converters 24R, 24G and 24B tothe correction circuit 3 by way of the low-pass filters 26R, 26G and26B, because the rising and falling edges thereof tend to become dull orrounded (that is, to lose their sharpness or abruptness) due to thefilter characteristics of the low-pass filters 25R, 25G, 25B, 26R, 26Gand 26B, as shown by broken lines at (A) in FIG. 6. For this reason,with the dark current correction signal former 14 of the presentembodiment, when reading out the above-mentioned horizontal black darkcurrent correction data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and white darkcurrent correction data D[W_(RSH) /W_(GSH) W_(BSH) ]_(H) from theworking memory 18, the reading start time of the leading data in eachline is advanced repeatedly into each successive blanking intervalT_(BLK) by a predetermined time interval or period T as shown at (B) inFIG. 6, so that the leading data in each line is read out from theworking memory 18 repeatedly during the intervals or periods T, in orderto ensure optimum correction by preventing the occurrence of a situationin which the adverse waveform distortion effects caused by the filtercharacteristics of the low-pass filters 25R, 25G, 25B, 26R, 26G and 26Bare displayed during the regular correction time interval T_(O).

Instead of controlling the reading out of data from the working memory18 as described above, it is possible to prevent waveform distortion dueto the filter characteristics of the low-pass filters 25R, 25G, 25B,26R, 26G and 26B by controlling the writing of data to the workingmemory 18.

A way in which such control of writing to the working memory 18 may beachieved will now be described with reference to FIG. 6.

When detecting the white dark current characteristics, data having thesame level as levels just behind (after) the blanking intervals T_(BLK)of the output signals from the A/D converters 12R, 12G and 12B arewritten into the working memory 18 during the periods T. White darkcurrent correction data are thus stored in (written into) the workingmemory 18 during the periods T_(O).

When imaging, that is when picking up an image, data read out from anaddress or addresses of the working memory 18 corresponding to theperiods T and the white dark current correction data as read out from anaddress or addresses of the working memory 18 corresponding to theperiods T_(O) are supplied to the low-pass filters 26R, 26G and 26B viathe D/A converters 24R, 24G and 24B. In this way, it is possible toprevent waveform distortion caused by the filter characteristics of thelow-pass filters 26R, 26G and 26B.

The control of writing data to and reading data from the working memory18 is performed by the system controller 27.

Black dark current correction may be carried out in the above case inthe same way as that in which white dark current correction is effected.

With the above-described shading correction apparatus of the presentembodiment, dark current correction data for each of the imaging devices1R, 1G and 1B are formed from the imaging output data D_(R), D_(G) andD_(B), digitized by the A/D converters 12R, 12G and 12B of the R, G andB channels from the imaging output signals E_(R), E_(G) and E_(B) fromthe first, second and third imaging devices 1R, 1G and 1B as lightexposure controlled by the iris unit 5, and the so-formed dark currentcorrection data are stored in the working memory 18 constituted by aRAM. Then, during actual imaging, that is when dark current correctionsignals for each of the imaging devices 1R, 1G and 1B are formed on thebasis of the dark current correction data read out from the workingmemory 18 so that the imaging output signals E_(R), E_(G) and E_(B) fromthe imaging devices 1R, 1G and 1B may be automatically subjected to darkcurrent correction processing.

In addition, with the above-described dark current correction apparatus,the state of light exposure of the first, second and third imagingdevices 1R, 1G and 1B is controlled, on the one hand, by the iris unit 5so that no light will be incident on the imaging surfaces of the first,second and third imaging devices 1R, 1G and 1B and, under thethus-controlled state of light exposure, the black dark currentcorrection data D[B_(RSH) ], D[B_(GSH) ] and D[B_(BSH) ] for each of theimaging devices 1R, 1G and 1B are formed from the imaging output dataD_(R), D_(G) and D_(B) digitized from the imaging output signals E_(R),E_(G) and E_(B) of the imaging devices 1R, 1G and 1B by the A/Dconverters 12R, 12G and 12B of the R, G and B channels. On the otherhand, the state of light exposure of the first, second and third imagingdevices 1R, 1G and 1B is controlled by the iris unit 5 so that light ofuniform light intensity will be incident on the whole of the imagingsurfaces of the first, second and third imaging devices 1R, 1G and 1Band, under the thus-controlled state of light exposure, the white darkcurrent correction data D[W_(RSH) ], D[W_(GSH) ] and D[W_(BSH) ] foreach of the imaging device 1R, 1G and 1B are formed from the imagingoutput data D_(R), D_(G) and D_(B) of the R, G and B channels digitizedfrom the imaging output signals E_(R), E_(G) and E_(B) of the imagingdevices 1R, 1G and 1B. These black dark current correction dataD[B_(RSH) ], D[B_(GSH) ] and D[B_(BSH) ], as well as the white darkcurrent correction data D[W_(RSH) ], D[W_(GSH) ] and D[W_(BSH) ], arestored in the working memory 18. Then, during actual imaging, the blackdark current correction signals B_(RSH), B_(GSH) and B_(BSH), as well asthe white dark current correction signals W_(RSH), W_(GSH) and W_(BSH),are formed on the basis of the black current correction data D[B_(RSH)], D[B_(GSH) ] and D[B_(BSH) ] as well as the white current correctiondata D[W_(RSH) ], D[W_(GSH) ] and D[W_(BSH) ], read out from the workingmemory 18, so that the imaging output signals E_(R), E_(G) and E_(B) ofthe imaging devices 1R, 1G and 1B may be quickly and reliably subjectedto black current correction processing and white current correctionprocessing.

Also, with the above-described current correction apparatus, the imagingoutput data digitized from the imaging output signals E_(R), E_(G) andE_(B) of the first, second and third imaging devices 1R, 1G and 1B withno light falling on the respective imaging surfaces are processed intodot-sequential black current correction data D[B_(RSH) /B_(GSH) /B_(BSH)] with the data volume reduced to one eighth of the original datavolume. On the other hand, the imaging output data D_(R), D_(G) andD_(B) digitized from the imaging output signals E_(R), E_(G) and E_(B)with light of uniform intensity falling on the full imaging surfaces ofthe imaging devices are processed into the white current correction dataD[W_(RSH) /W_(GSH) /W_(BSH) ] with the data volume reduced to one eighthof the original data volume. This results in a reduced currentcorrection data volume. In addition, since these dot-sequential blackcurrent correction data D[B_(RSH) /B_(GSH) /B_(BSH) ] and white currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ] are stored collectively inthe working memory 18, the various current correction data necessary forshading correction of the imaging devices may be stored in a singlememory without the necessity of providing plural memory means forstoring black and white shading correction data for the imaging devices.

Further, with the current correction apparatus of the presentembodiment, since the level data of the imaging output signals E_(R),E_(G) and E_(B) of the pixels of the imaging devices 1R, 1G and 1B,digitized more for each of the imaging devices 1R, 1G and 1B from theimaging output data D_(R), D_(G) and D_(B) as digitized by the A/Dconverters 12R, 12G and 12B of the R, G and B channels, are integratedin the horizontal and vertical directions for forming the black currentcorrection data D[B_(RSH) /B_(GSH) /B_(BSH) ]_(H) and the white currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(H) consistent with thehorizontal current component as well as the black current correctiondata D[B_(RSH) /B_(GSH) /B_(BSH) ]_(V) and the white dark currentcorrection data D[W_(RSH) /W_(GSH) /W_(BSH) ]_(V) consistent with thevertical dark current component, as the dark current correction data,the data volume of the shading correction data used for shadingcorrection may be reduced to enable the dark current correction data tobe stored in a working memory 18 of reduced storage capacity.

In addition, with the above-described dark current correction apparatus,by providing the working memory 18 constituted by a RAM and the backupmemory 19 constituted by an EEPROM, as the storage means for storing thedark current correction data, it becomes possible to perform theoperation of dark current correction data generation or shadingcorrection on the basis of the dark current correction data with the aidof the working memory 18, while it becomes possible to store the darkcurrent correction data for a prolonged time using the backup memory 19.Also, since the dark current correction data stored in the backup memory19 are previously reduced in data volume so that the number of outputdata is less at the middle than at the margin of the imaging surfaces ofthe imaging devices, an EEPROM which has a lesser storage capacity, andthus is less expensive, may be employed.

The invention is not limited to the above-described embodiment. Forexample, instead of processing the imaging output signals E_(R), E_(G)and E_(B) of the first, second and third imaging devices 1R, 1G and 1Bby analog dark current correction processing by means of the correctioncircuit 3, as in the above-described embodiment, it is possible toprovide a digital dark current correction circuit downstream of the A/Dconverters 12R, 12G and 12B of the R, G and B channels and to supply theblack dark current correction data D[B_(RSH) ], D[B_(GSH) ] as well asthe white dark current correction data D[W_(RSH) ], D[W_(GSH) ] andD[W_(BSH) ] to the digital correction circuit by means of the dataselectors 20 and 21.

With the above-described dark current correction apparatus embodying theinvention, the level data of imaging output signals of a predeterminednumber of pixels, digitized by analog/digital converting means from theimaging output signals of the imaging devices controlled as regardslight exposure by light exposure control means, are stored as darkcurrent correction data in memory means, and shading correction signalsare formed on the basis of the dark current correction data read outfrom the storage means, during actual imaging, thereby enabling theimaging output signals of the imaging devices automatically to besubjected to dark current correction processing.

In addition, with the above-described dark current correction apparatusembodying the invention, the state of light exposure is controlled bythe light exposure control means such that no light will be incident onthe imaging surfaces of the imaging devices, and black dark currentcorrection data are produced from the imaging output data digitized fromthe imaging output signals of the imaging devices in the thus-controlledstate of light exposure, thereby enabling the imaging output signals ofthe imaging devices to be subject to black dark current correctionprocessing quickly and reliably during actual imaging.

Further, with the dark current correction apparatus according to theabove embodiment, the state of light exposure is controlled by the lightexposure control means so that light of uniform light intensity will beincident on the entirety of the imaging surfaces of the imaging devices,and white dark current correction data are produced from imaging outputdata digitized from imaging output signals of the imaging devices in thethus-controlled state of light exposure, thereby enabling the imagingoutput signals of the imaging devices to be subjected to white darkcurrent correction quickly and reliably during actual imaging.

Also, with the dark current correction apparatus according to the aboveembodiment of the invention, by providing the random access memory (RAM)and the electrically erasable programmable read only memory (EEPROM) asstorage means, the operation of forming the dark current correction dataor correcting the dark current by means of the dark current correctiondata may be performed using the RAM as a working memory, while the darkcurrent correction data may be stored for a prolonged time using theEEPROM as a backup memory. Further, the dark current correction datastored in the programmable read-only memory have been reduced in datavolume so that the number of output data is less at the middle than atthe margin of the imaging surface of each imaging device, so that itbecomes possible to use memory of a smaller storage capacity.

A shading correction apparatus according to a second embodiment of theinvention will now be described in detail with reference to FIGS. 7 to12 of the drawings. The apparatus of FIGS. 7 to 12 is similar in manyrespect to that of FIGS. 1 to 6.

The dark current correction circuit according to the present embodimentis operative to eliminate a shading component in an imaging outputsignal of an imaging device 101 having an array of a large number ofpixels arranged in a matrix configuration and, as shown in FIG. 7, theapparatus comprises a correction circuit 103 for processing the imagingoutput signal from the imaging device 101 by dark current correctionprocessing, an analog to digital (A/D) converter 105 for converting theoutput signal of the correction circuit 103 into a digital signal, and adark current correction signal forming section 110 for forming a darkcurrent correction signal (or signals) on the basis of output data fromthe A/D converter 105.

The imaging device 101 used with the present embodiment is a CCD imagesensor in which a number (M×N) of pixels S₁₁ to S_(MN) are arrayed in amatrix configuration comprising a number M of horizontal rows of pixelsand a number N of vertical columns of pixels. The imaging device 110 isdriven by a CCD driving unit (not shown) so that signal charges for thetotality of the pixels S₁₁ to S_(MN) constituting a field or frame isread out during one vertical scanning period. The sequential signalcharges read out from the imaging device 101 are supplied as an imagingoutput signal via a preamplifier 102 to the correction circuit 103.

An iris unit 109 controlled by an iris control circuit or driver 108,which is in turn controlled by a system controller 107, is providedahead of an imaging surface of the imaging device 101.

The correction circuit 103 subjects the imaging output signal from theimaging device 101 to dark current correction processing by using thedark current correction signal supplied by the dark current correctionsignal forming section 110, and is constituted by, for example, asubtractor (not shown) for subtracting the dark current correctionsignal from the imaging output signal and a divider (not shown) fordividing the imaging output signal by the dark current correctionsignal.

The output signal from the correction circuit 103 is supplied via apreknee circuit 104 to the A/D converter 105. The preknee circuit 104effects non-linear processing of the output signal from the correctioncircuit 103 to prevent the input signal level to the A/D converter 105exceeding the dynamic range thereof.

The A/D converter 105 processes the output signal of the correctioncircuit 103, as supplied via the preknee circuit 104, to form level dataindicating the signal level of the output signal. The level dataproduced by the A/D converter 105 are supplied as dark current correctedimaging output data to the dark current correction signal formingsection 110, and to downstream signal processing circuits (not shown),via a defect correction circuit 106.

The defect correction circuit 106 processes signal charges, that isparts of the imaging output signal, from defective pixels of the imagingdevice 101, by defect correction processing that corrects the signallevel of the imaging output signal. The circuit 106 performs the defectcorrection processing on the basis of previously detected defectivepixel data of the imaging device 101.

The dark current correction signal forming circuit 110 of the presentembodiment is constituted by: a sampling circuit 111 for sampling leveldata supplied from the A/D converter 105 via the defect correctioncircuit 106 at a predetermined sampling interval; an arithmetic unit ordevice or central processor unit (CPU) 112 supplied with sampled datafrom the sampling circuit 111; a memory 113 connected to the arithmeticdevice 112; sawtooth (serrated) signal generators 114H, 114V forgenerating sawtooth signals at the horizontal scanning period and thevertical scanning period, respectively; parabolic signal generators115H, 115V for generating parabolic signals at the horizontal scanningperiod and the vertical scanning period, respectively; level controllers(level control circuits) 116H, 116V for controlling the signal levels ofthe sawtooth signals from the sawtooth signal generators 114H, 114V;level controllers (level control circuits) 117H, 117V for controllingthe signal levels of the parabolic signals from the parabolic signalgenerators 115H, 115V; and adders 118H, 118V and 118 for summingtogether the sawtooth signals and the parabolic signals.

The dark current correction signal forming section 110 is controlled bythe system controller 107 to be switched between a dark currentdetection mode of operation and a dark current correcting mode ofoperation.

For black dark current detection during the shading detecting mode ofoperation, the iris unit 109 provided ahead of the imaging surface ofthe imaging device 101 is closed by the iris control circuit 108 (inturn controlled by the system controller 107) so that an imagingoperation is performed without any light falling on the imaging surface.For white dark current detection, the iris unit 109 is opened by theiris control circuit 108 (in turn controlled by the system controller107) and an imaging operation is performed under a condition such that,with the use of a white pattern, such as a Porta pattern, light ofuniform light intensity will be incident on all of the imaging surface.The imaging output signal obtained from the imaging device 101 duringthe dark current detecting mode is supplied via the preknee circuit 104to the A/D converter 105 in an uncorrected state, that is without beingsubjected to dark current correction processing by the correctioncircuit 103.

The sampling circuit 111 processes the level data supplied from the A/Dconverter 105 via the defect correction circuit 106 by sampling, fromthe imaging output signal constituted by the signal charges of thepixels S₁₁ to S_(MN) of the imaging device 101, level data of theimaging output signal from the signal charges of a number (m×n) ofpixels composed of a number m of horizontal rows of pixels and a numbern of vertical columns of pixels, as shown by hatching in FIG. 8, andcumulatively adding the level data of the imaging output signal forpixels lying at the same positions P_(h1) to P_(hm) in the horizontaldirection, thereby forming a first data string D[1_(h1) to 1_(hm) ]indicative of dark current characteristics in the horizontal direction,and also cumulatively adding the level data of the imaging output signalfor pixels lying at the same positions P_(v1) to P_(vm) in the verticaldirection, thereby forming a second data string D[1_(v1) to 1_(vm) ]indicative of dark current characteristics in the vertical direction.

The signal to noise (S/N) ratio of the level data may be improved byusing a cumulative sum of level data of the imaging output signalcorresponding to the pixels lying at the same positions in thehorizontal or vertical direction. The S/N ratio of the level data may befurther improved by synchronized addition on a frame-by-frame basis.

It is to be noted that, in the present embodiment, the sampling circuit111 forms first and second data strings D[1_(h1) to 1_(hm) ]_(B) andD[1_(v1) to 1_(vm) ]_(B), indicative of black dark currentcharacteristics detected from the imaging output signal obtained fromthe imaging device 101 by carrying out imaging without any light fallingon the imaging surface, and forms first and second data strings D[1_(h1)to 1_(hm) ]_(W) and D[1_(v1) to 1_(vm) ]_(W), indicative of white darkcurrent characteristics detected from the imaging output signal obtainedby carrying out the imaging operation with light of uniform lightintensity incident on all of the imaging surface, and transmits thesedata strings to the arithmetic device 112 for storage in the memory 113.

The arithmetic device 112 calculates, from the black dark currentcharacteristics indicated by the first and second data strings D[1_(h1)to 1_(hm) ]_(B) and D[1_(v1) to 1_(vm) ]_(B) stored in the memory 113, aquadratic curve indicating a dark current waveform in the horizontaldirection and another quadratic curve indicating a dark current waveformin the vertical direction. The arithmetic device 112 also calculates,from the white dark current characteristics indicated by the first andsecond data strings D[1_(h1) to 1_(hm) ]_(W) and D[1_(v1) to 1_(vm)]_(W) stored in the memory 113, a quadratic curve indicating a darkcurrent waveform in the horizontal direction and another quadratic curveindicating a dark current waveform in the vertical direction.

The arithmetic operation performed by the arithmetic device 112 to findthe quadratic curves may comprise fitting a quadratic curve (y=ax²+bx+c) to the data strings obtained in the above-described manner by thesampling circuit 111 by a least squares method.

Thus, the sum of the squares of the deviations or errors E when thecurve y=ax² +bx+c is fitted to a number r of waveform sampling data (x₁,y₁), (x₂, y₂), . . . , (x_(r), y_(r)) is given by ##EQU1##

For minimizing the sum of the squares of the errors E, it suffices if##EQU2## that is if the results of partial differentiation of E withrespect to coefficients a, b and c are equal to zero.

The coefficients a and b may be represented by the equations ##EQU3##

The coefficients C₀ to C₆ in Equations (3) and (4) are given by:##EQU4##

If an equal sampling interval is used, and x_(i) is normalized, x_(i)=i, so that the coefficients C₀ to C₆ in Equations (3) and (4) becomefunctions of n. With a sampling interval of s, the coefficients a and bmay be expressed by the equations ##EQU5##

The coefficients C₀ to C₇ in Equations (12) and (13) above have beenreduced and, for the members r of the sampling data, assume for examplethe values shown in the following table:

                                      TABLE                                       __________________________________________________________________________    number                                                                        of data                                                                            coefficients                                                             n    C.sub.0                                                                           C.sub.1                                                                           C.sub.2                                                                           C.sub.3                                                                          C.sub.4                                                                           C.sub.5                                                                            C.sub.6                                                                           C.sub.7                                      __________________________________________________________________________    1    0   0   0   0  0   0    0   0                                            2    0   6   -9  3  0   -18  27  -9                                           3    2   10  -12 3  2   -42  49  -12                                          4    4   5   -5  1  20  -135 129 -25                                          5    14  7   -6  1  70  -231 187 -30                                          6    112 28  -21 3  560 -1092                                                                              767 -105                                         7    84  12  -8  1  84  -108 67  -8                                           8    168 15  -9  1  168 -153 85  -9                                           9    924 55  -30 3  4620                                                                              -3135                                                                              1577                                                                              -150                                         10   528 22  -11 1  2640                                                                              -1386                                                                              637 -55                                          11   858 26  -12 1  1430                                                                              -598 253 -20                                          12   4004                                                                              91  -39 3  4004                                                                              -1365                                                                              535 -39                                          13   2002                                                                              35  -14 1  2002                                                                              -567 207 -14                                          14   2912                                                                              40  -15 1  14560                                                                             -3480                                                                              1189                                                                              -75                                          15   12376                                                                             136 -48 3  61880                                                                             -12548                                                                             4061                                                                              -240                                         __________________________________________________________________________

The arithmetic device 112 calculates coefficients a_(HB) and b_(HB) ofthe quadratic curve fitted to the horizontal dark current waveform ofthe black dark current characteristics, indicated by the first andsecond data strings D[l_(h1) to l_(hm) ]_(B) and D[l_(v1) to l_(vm)]_(B) stored in the memory 113, and coefficients a_(VB) and b_(VB) ofthe quadratic curve fitted to the vertical dark current waveform of theblack shading characteristics, and stores these coefficients a_(HB),b_(HB), a_(VB) and b_(VB) in the memory 113. Also, the arithmetic device112 calculates coefficients a_(HW) and b_(HW) of the quadratic curvefitted to the horizontal dark current waveform of the white dark currentcharacteristics, indicated by the first and second data strings D[l_(h1)to l_(hm) ]_(W) and D[l_(v1) to l_(vm) ]_(W) stored in the memory 113,and coefficients a_(VW) and b_(VW) of the quadratic curve fitted to thevertical dark current waveform of the white shading characteristics, andstores these coefficients a_(HW), b_(HW), a_(VB) and b_(VB) in thememory 113.

In this manner, during the dark current detecting mode of operation,that is the mode of detecting the dark current characteristics, thearithmetic device 112 calculates the coefficients a_(HB), b_(HB), a_(VB)and b_(VB) of the quadratic curves fitted to the shading waveforms inthe horizontal and vertical directions of the black dark currentcharacteristics, and the coefficients a_(HW), b_(HW), a_(VW) and b_(VW)of the quadratic curves fitted to the dark current waveforms in thehorizontal and vertical directions of the white dark currentcharacteristics, and stores these coefficients in the memory 113.

When the system controller then causes the dark current correcting modeof operation, that is the mode of correcting the dark currentcharacteristics, to be entered, the arithmetic unit 112 reads out fromthe memory 113 the coefficients a_(h), b_(H), a_(V) and b_(V) stored inthe memory 113 during operation of the mode of detecting the darkcurrent characteristics, and transmits these coefficients to the levelcontrollers 116H, 117H, 116V and 117V.

The level controller 116H processes the sawtooth signal for thehorizontal scanning period, outputted by the sawtooth signal generator114H, in accordance with a level control operation consistent with thecoefficient b_(H) of the first order term of the quadratic curve fittedto the dark current waveform in the horizontal direction of the darkcurrent characteristics.

The level controller 117H processes the parabolic signal of thehorizontal scanning period, outputted by the parabolic signal generator115H, in accordance with a level control operation consistent with thecoefficient a_(H) of the second order term of the quadratic curve fittedto the dark current waveform in the horizontal direction.

The level controller 116V processes the sawtooth signal for the verticalscanning period, outputted from the sawtooth signal generator 114V, inaccordance with a level control operation consistent with thecoefficient b_(V) of the first order term of the quadratic curve fittedto the dark current waveform in the vertical direction of the darkcurrent characteristics.

The level controller 117V processes the parabolic signal for thevertical scanning period, outputted by the parabolic signal generator115V, in accordance with a level control operation consistent with thecoefficient a_(V) of the second order term of the quadratic curve fittedto the dark current waveform in the vertical direction.

The adder 118H sums together the sawtooth signal processed by the levelcontrol circuit 116H in accordance with the level control operationconsistent with the coefficient b_(H), and the parabolic signalprocessed by the level control circuit 117H in accordance with the levelcontrol operation consistent with the coefficient a_(H), to form a darkcurrent correction signal for the horizontal direction. The adder 118Vsums together the sawtooth signal processed by the level control circuit116V in accordance with the level control operation consistent with thecoefficient b_(V), and the parabolic signal processed by the levelcontrol circuit 117V in accordance with the level control operationconsistent with the coefficient a_(V), to form a dark current correctionsignal for the vertical direction. The adder 118 sums the horizontal andvertical dark current correction signals, formed as described above bythe adders 118H and 118V, respectively, to form a dark currentcorrection signal for the entire frame. The dark current correctionsignal thereby formed by the adder 118 is supplied to the correctioncircuit 103.

In the present embodiment, the dark current correction signal formingsection 110 includes a processing system for forming a black darkcurrent correction signal on the basis of the coefficients a_(HB),b_(HB), a_(VB) and b_(VB) of the curves approximating to the black darkcurrent waveform, and a processing system for forming a white darkcurrent correction signal on the basis of the coefficients a_(HW),b_(HW), a_(VW) and b_(VW) of the curves approximating to the white darkcurrent waveform. The black dark current correction signal and the whitedark current correction signal are supplied to the correction circuit103.

The correction circuit 103 processes the imaging output signal suppliedby the imaging device 101 via the preamplifier 102 by adding theabove-mentioned black dark current correction signals thereto to performblack dark current correction. The circuit 103 also processes theimaging output signal supplied by the imaging device 101 by multiplyingit by the white dark current correction signal to perform white darkcurrent correction. The imaging output signal thus subjected by thecorrection circuit 103 to black dark current correction processing andwhite dark current correction processing is digitized by the A/Dconverter 105 so as to be supplied as dark current corrected imagingoutput data to the downstream signal processing circuits (not shown).

It is to be noted that the circuit construction of the sawtooth signalgenerators 114H, 114V, as well as of the parabolic signal generators115H, 115V, would be simpler if use were made of a digital design or aconfiguration other than of generators used in an analog design or aconfiguration using operational amplifiers or gain control amplifiers.

A digital design of the sawtooth signal generators 114H and 114V may beachieved by using counters. For example, as shown in FIG. 9, a digitaldesign for each of the sawtooth generators 114H and 114V may be achievedby using first and second counters 121, 122, a subtraction circuit(subtractor) 123 and an exclusive OR circuit 124. The counters 121, 122of the sawtooth signal generators are reset (at reset inputs R thereof)by reset pulses at the repetition period of the output sawtooth signalto count clock pulses X at a repetition period corresponding to thepixel pitch. That is, for forming the sawtooth signal for the horizontalscanning period, the counters 121, 122 are reset by rest pulses at thehorizontal scanning period for counting clock pulses at a repetitionperiod corresponding to the pixel pitch in the horizontal direction. Forforming the sawtooth signal at the vertical scanning period, thecounters 121, 122 are reset by reset pulses at the vertical scanningperiod for counting clock pulses at a repetition period corresponding tothe pixel pitch in the vertical direction, that is the horizontalscanning period.

The first counter 121 has a carry output terminal (CRY) thereofconnected to a load input terminal (LOAD) thereof, and coefficient datasupplied to a data input terminal (DATA) thereof are loaded at thetiming of the carry output. The coefficient data represent the gradientor inclination of the output sawtooth signal. The coefficients b_(HB),b_(VB), b_(HW) and b_(VW), calculated by the arithmetic device 112during the mode of detecting the shading characteristics, are used asthe coefficient data. The first counter 121 transmits the carry outputto an enable input terminal (EN) of the second counter 122 at arepetition period consistent with the loaded coefficient data.

The second counter 122 counts the clock pulses X each time a carryoutput of the first counter 121 is supplied to the enable input terminalof the counter 122 to output sawtooth signal data y_(SW) having agradient 1/b corresponding to the coefficient data b. The sawtoothsignal data y_(SW), formed by the second counter 122, are supplied tothe subtractor 123, in which the value n/2b is subtracted from the datay_(SW) to remove the d c component and provide a sawtooth signal outputas shown in FIG. 10 (FIG. 10 shows two such signals, namely one for b=1,represented by a full line, and one for b=2, represented by a dashedline.)

The above-mentioned value n stands for the total number of pixels perscanning line when the sawtooth signal at the horizontal scanning periodis to be formed, and the total number of pixels in each frame when thesawtooth signal at the vertical scanning period is to be formed.

The sawtooth signal data y_(SWO), thus freed of the d c component by thesubtractor 123, and represented by ##EQU6## is outputted, after polaritydata are provided thereto in the exclusive OR circuit 124.

A digital configuration or design for each of the parabolic signalgenerators 115H, 115V may be achieved, as shown by way of example inFIG. 11, by using a sawtooth signal generator 131, an integratingcircuit 132 and a subtraction circuit 133.

The sawtooth signal generator 131 is constituted by a first counter 134,which has a data input terminal (DATA) thereof supplied with coefficientdata representing the gradient of the output sawtooth signal, and asecond counter 135, which has an enable input terminal (EN) thereofsupplied with a carry output (CRY) of the first counter 134. Thecounters 134, 135 are reset (at reset inputs R thereof) by reset pulsesat the repetition period of the parabolic signals generated by theparabolic signal generator of digital configuration to count clockpulses X at a repetition period corresponding to the pixel pitch. Thus,when forming the parabolic signal at the horizontal scanning period, thecounters 134, 135 are reset by reset pulses at the horizontal scanningperiod to count clock pulses of a repetition period corresponding to thehorizontal pixel pitch. On the other hand, when forming the parabolicsignal at the vertical scanning period, the counters 134, 135 are resetby rest pulses at the vertical scanning period to count clock pulses ofa repetition period corresponding to the pixel pitch in the verticaldirection, that is the vertical scanning period.

The first counter 134 has its carry output terminal (CRY) connected to aload input terminal (LOAD) thereof, and the input coefficient datasupplied to its data input terminal (DATA) is loaded at the timing ofthe carry output. The coefficient data, as mentioned above, representthe gradient or inclination of the output sawtooth signal. Thecoefficients a_(HB), a_(VB), a_(HW) and a_(VW), calculated by thearithmetic unit 112 during the mode of detecting the dark currentcharacteristics, are used as the coefficient data. The counter 134transmits the carry output to the enable input terminal (EN) of thesecond counter 135 at a repetition period consistent with the loadedcoefficient data.

The second counter 135 counts the clock pulses X each time a carryoutput of the first counter 124 is supplied to the enable input terminalof the counter 135 to output sawtooth signal data y_(SW), represented bythe equation ##EQU7## having a gradient 1/a consistent with thecoefficient data a.

The sawtooth signal outputted by the sawtooth signal generator 131 isshown by a straight line in FIG. 12. (More precisely, two such sawtoothsignals are shown in FIG. 12, namely one for a=1, represented by a fullstraight line, and one for a=2, represented by a dashed straight line.)

The integrating circuit 132 for integrating the sawtooth signal datay_(SW) from the sawtooth signal generator 131 is constituted by an adder(addition circuit) 136 supplied with the data y_(SW), a latch (latchcircuit) 137 for latching an addition output from the adder 136, and asubtractor (subtraction circuit) 139 supplied with the data y_(SW) via a1/2 (one half) multiplier 138. The latch 137 transmits latch output datathereof to the adder 136 and to the subtractor 139.

In operation of the above-described integration circuit 132, the adder136 adds the sawtooth signal data y_(SW) from the sawtooth signalgenerator 131 to the latch output data from the latch 137, and theaddition output is latched for cumulatively adding the sawtooth signaldata y_(SW). The subtractor 139 subtracts sawtooth signal data y_(SW)/2, supplied via the 1/2 multiplier 138, from the latch output data ofthe latch 137 (that is, the cumulative sum data of the sawtooth signaldata y_(SW)) to form parabolic signal data represent by the equationy_(pb) ##EQU8##

The parabolic signal data y_(pb) generated by the integrating circuit132 are supplied to the subtraction circuit 133, where the value n/3a issubtracted from the data y_(pb). Thus, parabolic signal data y_(pbo)freed of d c components, as represented by the equation ##EQU9## and asshown by a curved line in FIG. 12, are outputted. (More precisely, twosuch parabolic signals are shown in FIG. 12, namely one for a=1,represented by a full curved line, and one for a=2, represented by adashed curved line.)

The symbol n used in the above equations represents the total number ofpixels per scanning line and the total number of scanning lines perframe, according to whether the sawtooth signal for the horizontalscanning period or the sawtooth signal for the vertical scanning period,respectively, are being formed.

The sawtooth signal data y_(SWO) and the parabolic signal data y_(pbo),formed by the sawtooth signal generator and the parabolic signalgenerator of digital circuit configuration, respectively, may be summedtogether to provide a digital dark current correction signal or signals.That digital dark current correction signal or signals may be convertedby digital to analog converting means into a corresponding analog signalor signals and supplied to the correction circuit 103.

The correction circuit 103 may if desired be designed to have a digitalconfiguration and be provided downstream of the A/D converter 105 toperform the dark current correction in a digital fashion, though thisinvolves a complicated processing operation in the vicinity of the kneepoint in the preknee processing operation. In addition, the circuit isincreased in size as regards black dark current correction because asubtraction operation needs to be performed after the dark currentcorrection signal is multiplied by a gain after gain switching on theanalog side.

In the present embodiment, the shading correction signal forming section110 checks whether the dark current correction error in the dark currentcorrected imaging output data for the dark current correction mode isnot more than a predetermined value stored in the memory 113. If thedark current correction error is larger than the predetermined value,the signal forming section 110 causes the gains of the iris controlcircuit 108 and the preamplifier 102 to be changed to reduce the errorand, using these changed (renewed) gains, repeats the operation of thedark current detecting mode to store coefficient data which will give acorrection error not larger than the predetermined value. In this way,the imaging signal from the imaging device 101 may be subjected tooptimum dark current correcting processing.

With the above-described dark current correction apparatus embodying theinvention, black and/or white, dark current characteristics may bedetected by digital processing of imaging output signals obtained froman imaging device comprising an array of a large number of pixels in amatrix configuration with no light falling on the imaging device andwith light of uniform light intensity incident on all of an imagingsurface of the imaging device, according to which sawtooth signals andparabolic signals having the levels necessary for dark currentcorrection may be formed automatically. By sampling imaging outputsignal level data of the pixels of the imaging device at a predeterminedinterval in both the vertical and horizontal directions, dark currentcharacteristics of the imaging output signal of the imaging device maybe detected on the basis of first and second data strings each having areduced data amount or volume, so that the dark current characteristicsmay be detected using a memory of a reduced storage capacity.

Thus, in accordance with the present embodiment, which provides a darkcurrent correcting apparatus in which an imaging output signal from theimaging device is subjected to dark current correcting processing bymeans of sawtooth signals from sawtooth signal generators and parabolicsignals from parabolic signals generators, satisfactory dark currentcorrection may be performed quickly and reliably by a simple circuit.

References in the foregoing description and in the appended claims to adark current signal component being eliminated from an output signal ofan image pick-up device (imaging device) are to be construed as coveringboth complete and partial elimination.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

We claim:
 1. A dark current correction apparatus for eliminating a darkcurrent signal component from an output signal of an image pick-updevice having a plurality of pixels, the apparatus comprising:exposurecontrol means for controlling an exposure level of said image pick-updevice; analog to digital converting means for converting an outputsignal of said image pick-up device into a digital video signal; memorymeans for memorizing level data of said digital video signal outputtedfrom pixels of said image pick-up device while said exposure controlmeans is operative to control said exposure level to be at zero level;dark current correction signal generating means for generating a darkcorrection signal according to data read out from said memory means; andsignal processing means for processing an output signal produced by saidimage pick-up device while said image pick-up device is exposed to pickup an image, according to said dark current correction signal, so thatsaid dark current signal component is eliminated from said output signalproduced by said image pick-up device while said image pick-up device isexposed to pick up an image wherein a dark current correction apparatusfor eliminating a dark current signal component from an output signal ofan image pick-up device having a plurality of pixels, the apparatuscomprising: exposure control means for controlling an exposure level ofsaid image pick-up device; analog to digital converting means forconverting an output signal of said image pick-up device into a digitalvideo signal; memory means for memorizing level data of said digitalvideo signal outputted from predetermined ones of said pixels of saidimage pick-up device while said exposure control means is operative tocontrol said exposure level to be at zero level; dark current correctionsignal generating means for generating a dark current correction signalaccording to data read out from said memory means; and signal processingmeans for processing an output signal produced by said image pick-updevice while said image pick-up device is exposed to pick up an image,according to said dark current correction signal, so that said darkcurrent signal component is eliminated from said output signal producedby said image pick up device while said image pick-up device is exposedto pick up an image, and wherein said memory means is operative tomemorize level data at predetermined periods adjacent to periods inwhich dark current correction data are memorized in said memory means,the level of said level data being the same as the levels of saiddigital video signal just after blanking intervals of said digital videosignal.
 2. Apparatus according to claim 1, wherein said signalprocessing means comprises subtracting means for subtracting said darkcurrent correction signal from said output signal produced by said imagepick-up device while said image pick-up device is exposed to pick up animage.
 3. A dark current correction apparatus for eliminating a darkcurrent signal component from an output signal of an image pick-updevice having a plurality of pixels, the apparatus comprising:exposurecontrol means for controlling an exposure level of said image pick-updevice; analog to digital converting means for converting an outputsignal of said image pick-up device into a digital video signal; memorymeans for memorizing level data of said digital video signal outputtedfrom pixels of said image pick-up device while said exposure controlmeans is operative to control said exposure level to be at zero level;dark current correction signal generating means for generating a darkcorrection signal according to data read out from said memory means; andsignal processing means for processing an output signal produced by saidimage pick-up device while said image pick-up device is exposed to pickup an image, according to said dark current correction signal, so thatsaid dark current signal component is eliminated from said output signalproduced by said image pick-up device while said image pick-up device isexposed to pick up an image, wherein a dark current correction apparatusfor eliminating a dark current signal component from an output signal ofan image pick-up device having a plurality of pixels, the apparatuscomprising: exposure control means for controlling an exposure level ofsaid image pick-up device; analog to digital converting means forconverting an output signal of said image pick-up device into a digitalvideo signal; memory means for memorizing level data of said digitalvideo signal outputted from predetermined ones of said pixels of saidimage pick-up device while said exposure control means is operative tocontrol said exposure level to be at zero level; dark current correctionsignal generating means for generating a dark current correction signalaccording to data read out from said memory means; and signal processingmeans for processing an output signal produced by said image pick-updevice while said image pick-up device is exposed to pick up an image,according to said dark current correction signal, so that said darkcurrent signal component is eliminated from said output signal producedby said image pick up device while said image pick-up device is exposedto pick up an image, and wherein said dark current correction signalgenerating means is operative repeatedly to read out the leading data ineach line from said memory means during predetermined periods ofblanking intervals of said digital video signal.
 4. A dark currentcorrection apparatus for eliminating a dark current signal componentfrom an output signal of an image pick-up device having a plurality ofpixels, the apparatus comprising:exposure control means for controllingan exposure level of said image pick-up device; analog to digitalconverting means for converting an output signal of said image pick-updevice into a digital video signal; memory means for memorizing leveldata of said digital video signal outputted from pixels of said imagepick-up device while said exposure control means is operative to controlsaid exposure level to be at zero level; dark current correction signalgenerating means for generating a dark correction signal according todata read out from said memory means; and signal processing means forprocessing an output signal produced by said image pick-up device whilesaid image pick-up device is exposed to pick up an image, according tosaid dark current correction signal, so that said dark current signalcomponent is eliminated from said output signal produced by said imagepick-up device while said image pick-up device is exposed to pick up animage, wherein a dark current correction apparatus for eliminating adark current signal component from an output signal of an image pick-updevice having a plurality of pixels, the apparatus comprising: exposurecontrol means for controlling an exposure level of said image pick-updevice; analog to digital converting means for converting an outputsignal of said image pick-up device into a digital video signal; memorymeans for memorizing level data of said digital video signal outputtedfrom predetermined ones of said pixels of said image pick-up devicewhile said exposure control means is operative to control said exposurelevel to be at zero level; dark current correction signal generatingmeans for generating a dark current correction signal according to dataread out from said memory means; and signal processing means forprocessing an output signal produced by said image pick-up device whilesaid image pick-up device is exposed to pick up an image, according tosaid dark current correction signal, so that said dark current signalcomponent is eliminated from said output signal produced by said imagepick up device while said image pick-up device is exposed to pick up animage, wherein said dark current correction signal generating meanscomprises: interpolating means for interpolating data read out from saidmemory means; digital to analog converting means for converting anoutput signal of said interpolating means into an analog signal; andlow-pass filter means connected to an output of said digital to analogconverting means for outputting said dark current correction signal. 5.A defective pixel correction apparatus for eliminating a defective pixelsignal component from an output signal of an image pick-up device havinga plurality of pixels, the apparatus comprising:exposure control meansfor controlling an exposure level of said image pick-up device; analogto digital converting means for converting an output signal of saidimage pick-up device into a digital video signal; memory means formemorizing level data of said digital video signal outputted frompredetermined ones of said pixels of said image pick-up device whilesaid exposure control means is operative to control said exposure levelsuch that said image pick-up device is uniformly exposed; defectivepixel correction signal generating means for generating a defectivepixel correction signal according to data read out from said memorymeans; signal processing means for processing an output signal producedby said image pick-up device while said image pick-up device is exposedto pick up an image, according to said defective pixel correctionsignal, so that said defective pixel signal component is eliminated fromsaid output signal produced by said image pick-up device while saidimage pick-up device is exposed to pick up an image, and wherein saidmemory means is operative to memorize level data at predeterminedperiods adjacent to periods in which defective pixel correction data arememorized in said memory means, the level of said level data being thesame as the levels of said digital video signal just after blankingintervals of said digital video signal.
 6. Apparatus according to claim5, wherein said signal processing means comprises dividing means fordividing said output signal, produced by said image pick-up device whilesaid image pick-up device is exposed to pick up an image, by saiddefective pixel correction signal.
 7. A defective pixel correctionapparatus for eliminating a defective pixel signal component from anoutput signal of an image pick-up device having a plurality of pixels,the apparatus comprising:exposure control means for controlling anexposure level of said image pick-up device; analog to digitalconverting means for converting an output signal of said image pick-updevice into a digital video signal; memory means for memorizing leveldata of said digital video signal outputted from predetermined ones ofsaid pixels of said image pick-up device while said exposure controlmeans is operative to control said exposure level such that said imagepick-up device is uniformly exposed; defective pixel correction signalgenerating means for generating a defective pixel correction signalaccording to data read out from said memory means; signal processingmeans for processing an output signal produced by said image pick-updevice while said image pick-up device is exposed to pick up an image,according to said defective pixel correction signal, so that saiddefective pixel signal component is eliminated from said output signalproduced by said image pick-up device while said image pick-up device isexposed to pick up an image, and wherein said defective pixel correctionsignal generating means is operative repeatedly to read out the leadingdata in each line from said memory means during predetermined periods ofblanking intervals of said digital video signal.
 8. A defective pixelcorrection apparatus for eliminating a defective pixel signal componentfrom an output signal of an image pick-up device having a plurality ofpixels, the apparatus comprising:exposure control means for controllingan exposure level of said image pick-up device; analog to digitalconverting means for converting an output signal of said image pick-updevice into a digital video signal; memory means for memorizing leveldata of said digital video signal outputted from predetermined ones ofsaid pixels of said image pick-up device while said exposure controlmeans in operative to control said exposure level such that said imagepick-up device is uniformly exposed; defective pixel correction signalgenerating means for generating a defective pixel correction signalaccording to data read out from said memory means; signal processingmeans for processing an output signal produced by said image pick-updevice while said image pick-up device is exposed to pick up an image,according to said defective pixel correction signal, so that saiddefective pixel signal component is eliminated from said output signalproduced by said image pick-up device while said image pick-up device isexposed to pick up an image, wherein said defective pixel correctionsignal generating means comprises: interpolating means for interpolatingdata read out from said memory means; digital to analog converting meansfor converting an output signal of said interpolating means into ananalog signal; and low-pass filter means connected to an output of saiddigital to analog converting means for outputting said defective pixelcorrection signal.
 9. A dark current correction apparatus foreliminating a dark current signal component from an output signal of animage pick-up device having a plurality of pixels, the apparatuscomprising:exposure control means for controlling an exposure level ofsaid image pick-up device; analog to digital converting means forconverting an output signal of said image pick-up device into a digitalvideo signal; sampling means for sampling said digital video signal invertical and horizontal directions at predetermined intervals while saidexposure control means is operative to control said exposure level to beat zero level; quadratic curve generating means for generating first andsecond quadratic curves according to said digital video signal assampled in the vertical and horizontal directions; sawtooth signalgenerating means for generating a sawtooth signal; parabolic signalgenerating means for generating a parabolic signal; first level controlmeans for controlling the level of said sawtooth signal according tocoefficients of a second order term of said first and second quadraticcurves; second level control means for controlling the level of saidparabolic signal according to coefficients of a first order term of saidfirst and second quadratic curves; and signal processing means forprocessing an output signal produced by said image pick-up device whilesaid image pick-up device is exposed to pick up an image, according tooutput signals of said first and second level control means, so thatsaid dark current signal component is eliminated from said output signalproduced by said image pick-up device while said image pick-up device isexposed to pick up an image.
 10. A defective pixel correction apparatusfor eliminating a defective pixel signal component from an output signalof an image pick-up device having a plurality of pixels, the apparatuscomprising:exposure control means for controlling an exposure level ofsaid image pick-up device; analog to digital converting means forconverting an output signal of said image pick-up device into a digitalvideo signal; sampling means for sampling said digital video signal invertical and horizontal directions at predetermined intervals while saidexposure control means is operative to control said exposure level suchthat said image pick-up device is uniformly exposed; quadratic curvegenerating means for generating first and second quadratic curvesaccording to said digital video signal as sampled in the vertical andhorizontal directions; sawtooth signal generating means for generating asawtooth signal; parabolic signal generating means for generating aparabolic signal; first level control means for controlling the level ofsaid sawtooth signal according to coefficients of a second order term ofsaid first and second quadratic curves; second level control means forcontrolling the level of said parabolic signal according to coefficientsof a first order term of said first and second quadratic curves; andsignal processing means for processing an output signal produced by saidimage pick-up device while said image pick-up device is exposed to pickup an image, according to output signals of said first and second levelcontrol means, so that said defective pixel signal component iseliminated from said output signal produced by said image pick-up devicewhile said image pick-up device is exposed to pick up an image.